1. Field of the Invention
The present invention relates to a liquid crystal display. More particularly, the present invention relates to a liquid crystal display that can be easily manufactured and used as a display device with good visibility.
2. Description of the Related Art
Liquid crystal display devices now have various applications such as display screens or monitors for personal computers, factory automation systems, televisions, information terminals and airport control towers. Liquid crystal display devices can also be used for light bulbs of a liquid crystal projector.
Most liquid crystal display devices in use today are of twisted nematic (TN) type, in which a nematic liquid crystal is filled between two glass substrates. When no voltage is applied, liquid crystal molecules are aligned parallel to the substrate and the display appears “white”. When a voltage is applied, the liquid crystal molecules align with the resulting electric field. Regions where the electric field is applied appear “black”. Therefore, it is possible to achieve many intermediate levels of gray scale by controlling the voltage and, in turn, molecular orientation vectors.
The TN liquid crystal display has, however, a relatively narrow viewing angle due to the inherent twisting behavior of the liquid crystal molecules. This problem of narrow viewing angle is particularly prominent in the direction of tilt of the liquid crystal molecules for gray-scale images.
Some techniques have been proposed to improve the visibility of the liquid crystal display devices. For example, Japanese Patent Laid-open Nos. 4-261522 (corresponding to U.S. Pat. No. 5,229,873) and 6-43461 (corresponding to U.S. Pat. No. 5,309,264) disclose the use of more than one liquid crystal domains. As shown in FIG. 1A, the liquid crystal display disclosed comprises a color filter substrate 501 and a lower substrate. The color filter substrate 501 comprises a common electrode 502 and an alignment layer 503 laminated thereon. A cutout 517 is formed in the common electrode and the alignment layer. The lower substrate comprises a pixel electrode 504 and another alignment layer 503 laminated thereon. Homeotropically aligned liquid crystal molecules 508 are placed between the substrates. The substrates-liquid crystal combination is sandwiched between two crossed polarizers of which polarization axes are perpendicular to each other. When a voltage is applied, the electric fields at the edges of the cutout (referred to as fringe fields) are tilted and each pixel is divided into two or more liquid crystal domains. The characteristics of the domains are determined by the shape of the cutout(s) of, for example, rectangular or X-shaped as shown in FIGS. 1B and 1C.
It should be noted that Japanese Patent Laid-open No. 4-261522 provides a high-contrast characteristics by means of controlling the tilt direction of the liquid crystal molecules during application of voltage. Japanese Patent Laid-open No. 6-43461 uses optical compensation films when necessary to improve the viewing angle in the black mode.
It is also disclosed in the latter that the viewing angle can be improved by using a fringe field for dividing each pixel into two or more domains even in a twisted nematic liquid crystal display rather than a homeotropic liquid crystal display.
The common electrode having the cutout is produced by using microfabrication processes such as photoresist processing, which are not required for the manufacture of typical TN liquid crystal display devices. In addition, the upper and lower substrates should be positioned precisely. This is a particularly significant problem for active matrix liquid crystal display devices having a switching element such as a TFT.
In the manufacture of typical active matrix liquid crystal display devices, switching elements such as thin film diodes are formed on one transparent substrate. This means that the microfabrication is required only for the substrate on which the switching elements are formed. It is not necessary for the other substrate (which is typically referred to as a common electrode) to be subjected to such microfabrication. Instead, the electrode is formed over the entire surface of the substrate.
However, the above-mentioned cutout makes it necessary to use the microfabrication of the common electrode, increasing the number of processes. Furthermore, it is necessary to position the upper and lower substrates presicely after the microfabrication.
Japanese Patent Laid-open No. 10-333180 describes a configuration where a thin film transistor (TFT), a gate line, and a drain line are all placed beneath a pixel electrode in order to prevent the electric fields generated around the TFT, the gate line, and the drain line from affecting the fringe field produced by the cutout(s) in the common electrode. However, the components beneath the pixel electrode reduce the aperture ratio of the liquid crystal display.
Japanese Patent Laid-open No. 10-20323 (corresponding to U.S. Pat. No. 5,963,290) discloses a TN liquid crystal display comprising two substrates sandwiching an liquid crystal layer therebetween, the liquid crystal layer having two or more small regions, wherein one substrate has an aperture in which a second electrode is provided. A voltage applied to the second electrode produces a fringe field, changing the tilt direction of the liquid crystal molecules inside each pixel to produce a multi-domain structure. This produces the increased viewing angles. It is noted that the voltage application to the second electrode involves a special driving procedure. However, this procedure cannot be omitted. The multi-domain structure can only be achieved after a voltage is applied to the second electrode.
Japanese Patent Laid-open No. 5-113561 (corresponding to European Patent No. 0 538 796) describes a homeotropic alignment type liquid crystal display that uses a compensation film having a negative birefringence to compensate the change in angle-dependent birefringence of the liquid crystal with no applied voltage and quarter wave plates having positive and negative optical activities to ensure brightness of the display. This technique produces the increased viewing angles for a black image but the orientation of the liquid crystal molecules are not defined precisely. Not all pixels have desired characteristics of the domains and the display can sometimes appears lightly harsh. The viewing angle itself is not very satisfactory.
Japanese Patent No. 2947350 discloses to provide a protrusion or a cutout on or in the upper and lower substrates in order to separate the homeotropically aligned liquid crystal molecules in response to the application of a voltage. At least one of them is the protrusion. However, this requires that the both substrates are subjected to lithographic processing and the edges of the substrates should be positioned precisely.
International Patent Publication No. WO91/10936 (JP-T-5-505247) discloses an In-Plane-Switching (IPS) liquid crystal display wherein both electrodes are mounted on the same substrate and the liquid crystal molecules can rotate in the substrate plane. When a voltage is applied across the electrodes, the liquid crystal molecules align themselves with the electric field while remaining parallel with the substrates. With this technique, no liquid crystal molecules become anchored to the substrate during the application of a voltage. The difference of the viewing direction can be reduced and it is possible to provide a liquid crystal display with very low variation of birefringence over wide viewing angles. Drawbacks of this technique are associated with a small aperture ratio and a cell gap because a smaller cell gap requires a higher driving voltage.
On the other hand, Journal of Applied Physics, Vol. 45, No. 12 (1974) 5466 and Japanese Patent Laid-open No. 10-186351 disclose a homeotropic-aligned liquid crystal with positive dielectric anisotropy combined with the IPS technique. The liquid crystal molecules are caused to be lined up in a lateral direction along the electric fields parallel with the substrates. It is noted that the homeotropic-aligned liquid crystal molecules take on different tilt angles due to the directions of the electric fields. The resulting multi-domain directional alignment contributes to providing a liquid crystal display with wide viewing angles.
In the driving methods based on the IPS technique and the electric fields parallel with the substrate for the homeotropically aligned molecules, a color filter layer is disposed between the layer on which the liquid crystal is placed and the opposite substrate. The electric fields generated by the application of potential across the source electrode and the common electrode may badly affect the color filter layer and in turn the display properties especially when the switching cell has a TFT configuration. This is because the dyes used for the color filter contain contaminants such as a sodium ion. The electric fields applied to the color filter layer results in charge buildup thereon. The charge buildup on the color filter layer leads continuous application of unnecessary electric fields to the portion of the liquid crystal beneath the region with charge, causing color distortion of the filter.
Japanese Patent Laid-open No. 10-186330 proposes to use a photo-sensitive material to form a square wall. This arrangement is used as a basic unit to produce a pixel. The liquid crystal molecules with negative dielectric anisotropy are caused to be oriented in different directions to produce two or more domains within each pixel. However, it is necessary to use the photolithography to form the suitable wall for the orientation of the liquid crystal molecules. Such photolithography increases the number of manufacturing steps.
With respect to the above, an object of the present invention is to overcome the drawbacks and problems associated with the conventional liquid crystal display devices by means of providing a liquid crystal display having a high contrast over wide viewing angles without addition of a complicated manufacturing step such as photo-resist processing and advanced edge-positioning. In particular, the present invention is directed to reduce the color distortion of the liquid crystal display.
Another object of the present invention is to provide a simple and easy method for manufacturing a liquid crystal display.
A yet another object of the present invention is to provide a method for driving a liquid crystal display with which the viewing angle-associated characteristics and properties of the display can be exploited effectively.